- Magnetic fields are caused by motion of some kind involving charged or magnetic material, and arise in the universe on every scale from the atomic to the galactic. The Earth’s magnetic field results from the convection of the Earth’s core, which contains large amounts of iron. On a larger scale, whole galaxies are thought to have magnetic fields on the evidence of systematic alignment of interstellar particles, like so many compass needles under the influence of a weak magnet. This magnetism probably arises from the interaction of charged particles in electric fields. In the solar system, the Earth has much the most powerful magnetic field of the terrestrial planets, although Mercury also has a magnetic field and there is a weak spiral magnetic field throughout the solar system spreading outwards from the Sun itself. At the Earth’s orbit the solar magnetic field is thousands of times weaker than the Earth’s. Magnetic fields cause splitting of spectral lines and other spectral effects, allowing them to be detected and measured in many stars. They are especially strong in some types including the T Tauri stars.